> or if lens scratches or dust were detectable in the same spots on the photos, revealing the photos were taken by the same camera.
This seems pretty useless. The only scenario where it gives FB more information is where person A takes a picture, gives it to person B through some route other than FB and then person B posts it on FB. Any other scenario and FB does not need to compare dust specks.
I feel there is no reason to expect Facebook would be content with data hosted on just their platform. With camera fingerprinting, they could be relatively sure who you are on other platforms like Twitter, VK, Youtube etc in cases where the user doesn't use the same email/name.
Your saying people who use FB regularly share photos outside of FB and then upload someone else's photos to FB? I'm not saying it doesn't happen, just that it's not the normal path.
I think people who use FB generally share photos with their friends who are also on FB, through FB.
Not in my experience and I have many friends outside of tech circles, since I'm not living in a tech bubble. When people get together group photos happen, or photos that you want, made with somebody else's phone.
The result is basically dozens of photos out of which only the best 2 or 3 get shared on Facebook.
And in general Facebook is not how those photos get shared. Not even its Messenger because it has annoying size limits, which matter for videos. If I were to guess, out of Facebook's properties, WhatsApp is probably the most popular photo sharing app by volume ;-)
This seems pretty useless. The only scenario where it gives FB more information is where person A takes a picture, gives it to person B through some route other than FB and then person B posts it on FB. Any other scenario and FB does not need to compare dust specks.